Dynamically configurable mlc state assignment

ABSTRACT

Memory devices and methods are disclosed, such as those facilitating a data conditioning scheme for multilevel memory cells. For example, one such memory device is capable of inverting the lower page bit values of a complete page of MLC memory cells when a count of the lower page data values is equal to or greater than a particular value or a comparison of current levels compared with a reference current level is equal to or exceeds some threshold condition. Memory devices and methods are also disclosed providing a means for determining initial programming pulse conditions for a population of memory cells based on the number of lower page data values being programmed to a logical 0 or a logical 1 data state.

TECHNICAL FIELD

The present disclosure relates generally to memory devices, and in aparticular embodiment, the present disclosure relates to methods andapparatus for providing reconfigurable multilevel memory stateassignments.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications.Non-volatile memory is memory that can retain its stored data for someextended period without the application of power. Flash memory devicestypically use a one-transistor memory cell that allows for high memorydensities, high reliability, and low power consumption. Changes inthreshold voltage (Vt) of the cells, through programming or “writing” ofcharge storage nodes (e.g., floating gates or trapping layers or otherphysical phenomena), corresponds to the data “stored” on each cell. Bydefining two or more ranges of threshold voltages to correspond toindividual data values, one or more bits of data may be stored on eachcell (in some cases, this may also be referred to as storing “in” a cellor stored “by” a cell, but there is no distinction intended by suchdifferences in terminology). Memory cells storing one bit of data byutilizing two threshold voltage ranges are typically referred to asSingle Level Cell (SLC) memory cells. Memory cells storing more than onebit of data on a cell by utilizing more than two possible thresholdvoltage ranges are typically referred to as Multilevel Cell (MLC) memorycells, although MLC memory cells can refer to any cell that can be usedto store more than two threshold voltage ranges.

MLC technology permits the storage of more than one bit on a cell,depending on the quantity of threshold voltage ranges assigned to thecell and the stability of the assigned threshold voltage ranges duringthe lifetime operation of the memory cell. The number of thresholdvoltage ranges, which are sometimes referred to as Vt distributionwindows, used to represent a bit pattern comprised of N-bits is 2^(N).

FIG. 1 illustrates an example threshold voltage distribution 100 for apopulation of MLC memory cells. For example, a cell may have a Vt thatfalls within one of four different voltage ranges 102/104/106/108 of 200mV, each being used to represent a data state corresponding to arespective bit pattern comprised of two bits. Typically, a dead space110 (which is sometimes referred to as a margin) of 0.2V to 0.4V ismaintained between each range to keep the Vt ranges from overlapping. Ifthe threshold voltage of the cell is within the first of the four Vtranges 102, the cell in this case is storing a logical 11 state and istypically considered the erased state of the cell. If the voltage iswithin the second of the four Vt ranges 104, the cell in this case isstoring a logical 01 state. A voltage in the third range 106 of the fourVt ranges would indicate that the cell in this case is storing a logical00 state. Finally, a Vt residing in the fourth Vt range 108 indicatesthat a logical 10 state is stored on the cell.

Another characteristic of MLC memory cells which distinguishes it fromSLC memory is that the Vt ranges in MLC memory cells tend to be muchnarrower and closer together than in SLC memory cells. These narrowranges should be reliably maintained in order to prevent corruption ofthe data stored on the memory cells. MLC memory also generally requiresthat several high voltage pulses be applied to shift memory cellthresholds to the higher threshold voltage ranges such as those ranges106 and 108 corresponding to the 00 and 10 states. Typically, the mostpositive memory cell threshold voltage range (e.g., range 108) requiresthe greatest number and magnitude of programming pulses. As the numberand magnitude of these applied programming pulses increases in order toachieve a higher threshold voltage, undesirable effects can result onboth the memory cells being programmed (e.g., selected memory cells) andon unselected memory cells nearby. For example, the increased magnitudeof the programming pulses can add additional gate stress to the memorycells. The number and magnitude of the applied programming pulses canalso cause shifts, also referred to as program disturbs, in theprogrammed Vt state of nearby memory cells due to capacitive couplingeffects, for example.

Flash memory typically utilizes one of two basic architectures known asNOR flash and NAND flash. The designation is derived from the logic usedto read the devices. In NOR flash architecture, a column of memory cellsare coupled in parallel with each memory cell coupled to a data line,commonly referred to as a bit line. In NAND flash architecture, a columnof memory cells are coupled in series with only the first memory cell ofthe column coupled to a bit line.

Common uses for flash memory and other non-volatile memory includepersonal computers, personal digital assistants (PDAs), digital cameras,digital media players, digital recorders, games, appliances, vehicles,wireless devices, mobile telephones and removable memory modules, andthe uses for non-volatile memory continue to expand.

For the reasons stated above, and for other reasons that will becomeapparent to those skilled in the art upon reading and understanding thepresent specification, there is a need in the art for alternativeprogramming methods for programming MLC memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing threshold voltage distributions levels for aplurality of MLC memory cells.

FIG. 2 is a diagram showing an array of NAND memory cells according toan embodiment of the disclosure.

FIG. 3 is a diagram showing a threshold voltage distribution during aprogramming operation according to an embodiment of the disclosure.

FIG. 4 is a diagram of a flow chart according to an embodiment of thedisclosure.

FIG. 5 is a diagram of a flow chart according to an embodiment of thedisclosure.

FIG. 6 is a diagram showing an example of a plurality of memory cellsand their initial intended programming states according to an embodimentof the disclosure.

FIG. 7 is a diagram showing a plurality of memory cells along with theirintended programming states according to an embodiment of thedisclosure.

FIG. 8 is a diagram of a flow chart illustrating an internal data moveoperation according to an embodiment of the disclosure.

FIG. 9 is a functional block diagram of an electronic system having atleast one memory device according to an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following detailed description of the present embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the disclosure may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theembodiments of the invention, and it is to be understood that otherembodiments may be utilized and that electrical, mechanical or processchanges may be made without departing from the present disclosure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense.

Flash memory cells are typically arranged in arrays of memory cellscomprising logical rows (e.g. memory cells coupled to word lines) andcolumns (e.g. memory cells coupled to bit lines) as illustrated in FIG.2. A given row is typically divided up into what are referred to in theart as pages of data. For example, a page may be defined as being a2,048 byte data storage region plus an additional 64 byte overheadstorage region. Page sizes according to the embodiments of the presentdisclosure may vary from the 2,048 byte plus 64 bytes, for example. Apage could be comprised of an entire row of memory cells. In the case ofMLC memory, each memory cell of a given page can store multiple pages(e.g., bit values) of data. For example, in an MLC memory cell storingtwo bits of data, the memory cell could have what is often referred toas upper and lower page data. MLC memory cells storing three bits ofdata can have upper, middle, and lower page data stored on the cell.Typically, in a two bit MLC memory cell storing a bit pattern of XY(e.g. 11, 01, 00 or 10) the X bit is referred to as the upper page bitand the Y bit is referred to as the lower page bit. However, thedesignations of upper and lower are arbitrary as long as they areconsistently utilized in regards to the device.

Memory cells 232-238 sharing a common access line, such as a word line230 (which may consist of, e.g., commonly coupled control gates of aconventional flash memory cell) are subjected to programming at the sametime although potentially to different threshold voltages (e.g., datastates). Although some of the memory cells may be inhibited fromprogramming, they would still be subjected to the same control gatevoltages as uninhibited memory cells on the same word line. Memory cellson the same word line can inhibited from further programming bytechniques such as manipulation of bit line voltages as is known tothose skilled in the art, for example. NAND strings of flash memorycells are arranged in logical columns of multiple memory cells, eachcoupled drain to source as shown in FIG. 2 such as those located betweengates 242 and 250. A drain select gate 242 couples one end of the NANDstring to an associated bit line BL0 222. A source select gate 250couples the opposing end of the NAND string to a common source line 220.Each bit line 222-228 is further coupled to sensing devices 240 (e.g.,sense amplifiers whose detail is not shown.) Sense amplifiers and othersensing devices are known to those skilled in the art and are thereforenot discussed further in relation to various embodiments of the presentdisclosure. FIG. 2 illustrates four bit lines. However, many more bitlines with associated NAND strings can exist as indicated by bit linesBL0-BLn 222-228 as shown in FIG. 2.

Flash memory cells are generally erased in blocks wherein all thethreshold voltages of the memory cells in the block are returned to acommon state. This state is typically referred to as the ‘erased’ or 11state, as shown in FIG. 1. Flash memory cells are typically programmed alogical row at a time as indicated by the circled memory cells 232-238of FIG. 2. Many more than four memory cells 232-238 shown in FIG. 2 mayexist per row 230. Programming is accomplished by providing pulses of aprogramming voltage to the word line 230 coupled to the row of memorycells to be programmed 232-238. With each programming pulse that isapplied, the threshold voltages of the memory cells selected forprogramming are shifted by some amount. Once a memory cell has reachedits intended threshold voltage level the cell is inhibited from furtherprogramming by techniques such as manipulation of bit line voltages asis known to those skilled in the art. This process continues until thethreshold voltages for the memory cells have all reached their intendedrange 102-108 as shown in FIG. 1. As discussed above, the memory cellsof a row will likely store different data states and thus each memorycell will have different final programmed threshold voltages. Forexample, if the memory cell 234 of FIG. 2 is to be programmed to a 01data state and memory cell 238 is to be programmed to a 00 data state,then memory cell 238 will typically require more programming pulses toachieve its intended data state than memory cell 234. In addition, theprogramming voltage is typically increased after each application of aprogramming pulse. For example, a first programming pulse may be equalto 14V, the second programming pulse may be increased to 15V, the thirdpulse increased to 16V and so on. These programming pulses needed tocomplete programming of the memory cells 232-238 in this example cancause a number of issues. One issue is the more programming pulses thatneed to be applied increase the time that is required to completeprogramming of the memory cells for the row of memory being programmed.Further, these additional programming pulses can cause undesirableshifts, or ‘program disturb,’ in memory cells of the selected row, andpotentially adjacent rows, that have already achieved their intendedthreshold voltage level. As the programming potential of eachprogramming pulse increases, these disturb effects can also increase.The embodiments of the present disclosure provide methods and apparatusto mitigate these program disturb issues, reduce the number of appliedprogramming pulses, reduce the magnitude and duration of the appliedprogramming pulses and/or reduce the overall time needed to program arow of memory cells.

Programming of memory cells can be accomplished through a process knownas Fowler-Nordheim tunneling wherein charges originating in a channelregion of the memory cell are forced through an insulating layer wherethey are then trapped in a charge storage node (e.g., floating gate.)The more charges that are trapped in the charge storage node the higherthe threshold voltage for the memory cell will be. The rate of chargetunneling is dependent on the potential difference between theprogramming voltage applied to the control gate and the potential of thechannel region of the memory cell. Programming may also be accomplishedthrough direct injection of charge carriers.

It has been proposed in U.S. Pat. No. 6,292,868 (issued Sep. 18, 2001)that the power required to program a population of memory cells in anSLC memory device storing either a 1 or a 0 data state can be reduced byinverting all the programmed states of the memory cells (e.g., either 1or 0) if the number of memory cells to be programmed from the erasedstate exceeds (e.g., is above or greater than) a predeterminedpercentage of the total number of memory cells. This method thus invertsall bits of data acted upon by the method of the '868 patent. However,mere inversion of all bits based upon whether a number of memory cellsto be programmed exceeds a threshold value becomes nonsensical in an MLCdevice where inverted data patterns do not mean the difference betweenprogramming or inhibiting a memory cell.

An embodiment of the present disclosure will be discussed in part by wayof reference to FIG. 3. FIG. 3 illustrates threshold voltage shiftsaccording to an embodiment of the present disclosure. The distributions300 shown in FIG. 3 illustrate the possible intended distribution ofthreshold voltages after completing a given programming operation on arow of memory cells. Programming of lower pages of data are typicallydone followed by upper page programming. However, it is possible that aparticular memory cell will only experience a lower page programmingoperation or only experience an upper page programming operationdepending on the data value to be stored in the particular cell. Theshift operation (e.g., programming operation) 312 of FIG. 3 illustratesthe threshold voltage shift of memory cells undergoing a lower page dataprogramming operation. For example, in two bit MLC memory cells whereinthe lower page data bit is to be programmed, the memory cells thresholdvoltages are shifted from the 11 erased state to the X0 intermediatestate during the lower page programming operation 312. In the 312programming operation, the X bit is a ‘don't care‘bit. Memory cells atthe X0 state are then processed further based on the intended upper pagedata programming state of the memory cells. For example, a cellcurrently at the X0 state and having an intended upper page data valueof 1, will be further programmed 316 to reside in the 10 state (range310). Memory cells residing in the X0 state which have an intended upperpage data value of 0, will be further programmed 314, if required, toreside in the 00 state (range 308).

The upper pages of memory cells having a lower page data value of 1(e.g., erased value) are processed as shown with respect to theprogramming operation 318. For example, memory cells having an intendedupper page and lower page data value of 1 remain in the 11 (erased)state (range 302). Memory cells having an intended lower page data valueof 1 and an intended upper page data value of 0 are further processed318 to shift the threshold voltage to the 01 state (range 306).According to one or more embodiments and as shown in FIG. 3, the datavalues (e.g., 11, 01, 00, 10) are assigned to each range such that thedata values of adjacent ranges differ by only one digit (e.g., bit.)Thus, FIG. 3 illustrates a MLC programming distribution 300 aftercompletion of programming.

As discussed above, the further the threshold voltage of a memory cellneeds to be shifted, the more ever increasing in magnitude programmingpulses are needed to achieve the desired threshold voltages. Forexample, programming memory cells to range 310 will require a greatertotal number of programming pulses at a higher potential than memorycells to be programmed to the 306 range. The greater the number andmagnitude of programming pulses applied to a control gate increases thetime to complete the programming operation and further increases theprogram disturb effects on other memory cells near the cells beingprogrammed. One or more embodiments of the present disclosure addressthese issues by analyzing the intended programmed states of a number(e.g., a page) of MLC memory cells and dynamically assign stateassignments to reduce the number of memory cells being programmed to thehigher level data states. For example, according to various embodimentsthe state assignments can be dynamically assigned so as to reduce thenumber of memory cells undergoing a lower page programming operationsuch as 312. This results in a reduction of memory cells undergoing theprogramming operations 314, 316 and an increase of memory cellsundergoing the programming operation 318, for example. This has theeffect of an overall reduction in the number of memory cells beingprogrammed to data states above the intermediate point (e.g., midpoint)320 in one or more embodiments as illustrated in FIG. 3. Additionalembodiments may have different numbers of data states on either side ofpoint 320.

FIG. 4 is a process diagram according to an embodiment of the presentdisclosure. An entire page or a block of memory cells along with theirrespective inversion flag bits, as discussed subsequently, are erased400 in preparation for a future programming operation. Entire pages ofmemory cells are erased as the embodiment performs a programmingoperation on an entire page of memory cells as opposed to a partial pageprogramming operation. According to the embodiment illustrated in FIG.4, a page of MLC memory cell data (e.g., lower page of original sourcedata) is made available 402, for example by a connected host or otherprocessor, that is intended to be programmed in a memory device. Anexample of a memory device can be a memory array of MLC Flash memorycells as illustrated in FIG. 2. A count is then performed 404 on thereceived data to determine how many 0s exist to be programmed in thelower page of each MLC memory cell. As illustrated in FIG. 3, a lowerpage bit value of 0 indicates that a memory cell needs to be programmedto a higher range 308 or 310. Alternate embodiments may count the numberof 1 s that exist in the received data. The count is performed on data(e.g., lower page data) that is intended to be programmed to the samepage (e.g., same row) of memory cells. Various means can be employed toaccomplish the count operation 404 or equivalent determining factors. Adigital adder as is known in the art can be utilized to count the numberof 0s existing in lower page values of the page of data to be written.Such digital adders might consist of a serial adder, ripple carry adder,carry look ahead adder, Manchester adder, Kogge-Stone adder, carry saveadder or a carry select adder, for example. An alternate method ofdetermining the number of lower page data values to be programmedaccording to various embodiments of the present disclosure is to performa cumulative current measurement in the data cache of the memory device.The data cache (e.g., cache register) is discussed subsequently withrespect to FIG. 8. The cumulative current measurement is then comparedto a reference current value. If the cumulative current measurementexceeds the reference current, that would indicate that a number oflower page bits are to be programmed that justifies inverting the lowerpage bits of the data to be written.

Each page of the memory device has an inversion flag bit that isassociated with it. An inversion flag bit that is in a set condition(e.g., equal to a logical 1 or 0) indicates an inversion operation hasoccurred as described subsequently. According to one or moreembodiments, the inversion flag may be implemented by utilizing aplurality of bits instead of a single bit. A majority vote calculatorcan then be used to ensure high reliability of the inversion status ofthe data. A comparison is then made 406 to determine if the count of 0sis greater than a particular threshold count value. It will be notedthat the condition “greater than a threshold count value” may beequivalent to the condition “equal to or greater than a threshold countvalue” upon designating the appropriate threshold count values. Forexample, the condition “greater than a threshold count value of 49” isequivalent to the condition “equal to or greater than a threshold countvalue of 50.” The threshold count value may be provided as a fixednumber or may be expressed as a percentage, or fraction, of the numberof memory cells involved in the counting operation.

If the count is less than the threshold count value 408, the data iswritten to the MLC memory cells without further data conditioning 414.If it is found that the count exceeds the threshold count value 410,then the data undergoes further conditioning. For example, the lowerpage data values are inverted 412 (e.g., 1 to 0, 0 to 1) and aninversion flag bit associated with the now inverted data is then set 412to indicate the lower page data has been inverted. Different embodimentsmay also set the associated inversion flag bit 412 either before orconcurrently with the inversion of the lower page of data 412.

Following the inversion of the lower page of data and setting theinversion flag bit 412, the data along with its associated inversionflag bit is then written to the memory 414. Various embodiments canwrite the inversion flag bit following (e.g., appended to) the data orthe inversion flag bit may be stored in a different location in thememory device. For example, the flag bit may be stored in the additional64 byte storage region as part of a page of memory. Subsequent to theprogramming of the lower page of data 414, the corresponding upper pagedata (e.g., upper page data sharing the same wordline as lower page data414) is obtained 416 and programmed 418.

At some time following the write operation illustrated by reference toFIG. 4, a read operation of data in the memory device as illustrated byFIG. 5 may be requested 500. For example, a host controller or processorattached to the memory device which had previously supplied originaldata to be stored in the memory device may request a read operation ofthe data be performed 500. A determination is first made to determine ifthe requested data to be read is located in an upper or a lower page ofmemory 502. If the read request 500 is to read data from the upper page504 then a read operation of the upper page of data 506 is performed andthe read data is subsequently output 520. If the read request is to readdata from a lower page 508 the lower page is then read 510. Adetermination is made following the read operation of the lower page ofdata 510 to determine if the inversion flag corresponding to the readdata is set 512. In at least one embodiment according to variousembodiments of the present disclosure, the determination of the state ofthe inversion bit (e.g., set or not set) is made in parallel with thelower page read operation. If the inversion flag is set 514, the dataread from the lower page is inverted 518 and outputted 520. If theinversion flag is not set 516, the data read from the lower page isoutputted 520 without performing an inversion operation. Thus, data thathad been inverted according to various embodiments of the presentdisclosure has been restored to the initially intended state (e.g., itsoriginal or “source” state).

FIGS. 6 and 7 provide a graphical representation of an embodimentaccording to the present disclosure. Each square (e.g., 610/612) of eachhorizontal bar represents an MLC memory cell coupled to its respectiveword line 204-216, 230 similar to the word lines illustrated in FIG. 2.However, the embodiments are not limited to the number of memory cellsand word lines shown in FIGS. 6 and 7. Memory arrays according to thepresent disclosure can include many more memory cells and word linesthan shown in FIGS. 6 and 7. The shaded blocks 610 indicate a memorycell having an intended (e.g., original) lower page data value of 0. Inthis example, a lower page data value of 0 indicates a programmingoperation is needed as illustrated by the lower page programmingoperation 312 shown in FIG. 3. Non-shaded blocks 612 of FIGS. 6 and 7indicate memory cells wherein the intended lower page data value is a 1.These memory cells will either remain in the 11 state 302 or may beprogrammed 318 to the 01 state 306 as illustrated in FIG. 3. Althoughthe embodiment illustrated in FIGS. 6 and 7 have a designated thresholdcount value of 50%, any percentage, or equivalent means to determine athreshold count value has been exceeded, between approximately 50% and100%, are possible according to various embodiments such that digits ofthe data may be inverted if half or more of the cells are to require aprogramming operation. Embodiments employing a count of lower pagememory data values that do not require a programming operation (e.g.logical 1 value instead of logical 0) can utilize a threshold countvalue between 0% and approximately 50%. However, because the subsets ofmemory cells requiring or not requiring a program operation are mutuallyexclusive, determining a count of memory cells not requiring a programoperation inherently determines a count of memory cells requiring aprogram operation. Referring again to the example embodiment of FIG. 6and 7, a threshold of 50% is used for the count of memory cellsinitially intended to have lower page data values of a 0 data state.

Each row of memory cells illustrated in FIGS. 6 and 7 is comprised of 32memory cells. Thus, the lower page values of each memory cell in a page(e.g., a row or partial row) of cells having 16 or more shaded cellswill undergo an inversion operation using a threshold count value ofapproximately 50%. The result of this inversion operation on the rows ofmemory cells of FIG. 6 204-216, 230 according to one or more embodimentsis illustrated in FIG. 7. For example, WL7 204 of FIG. 6 has 25 cellsthat will require a lower page programming operation as indicated by theshaded cells. WL7 704 of FIG. 7 indicates that after the inversionoperation of the lower page bits according to the embodiments of thepresent disclosure, only 7 cells now need a lower page programmingoperation. It should also be noted that the inversion flag bit 718associated with WL7 704 has been set. In a further example, WL5 208 ofFIG. 6 indicates that 13 memory cells require a lower page programmingoperation be performed as indicated by the shaded cells. Because only 13memory cells of WL5 208 require a lower page programming operation andthe threshold count value (50% in this example) is 16, the lower pagebit values of WL5 208 do not undergo an inversion operation. This resultis illustrated by WL5 708 of FIG. 7 in that the same shaded cells appearin WL5 208 of FIG. 6 and WL5 708 of FIG. 7. The inversion flag bit ofWL5 720 has also not been set indicating that the data has not undergoneany conditioning, i.e., modification, from its original state. It shouldbe noted that with respect to FIGS. 6 and 7, that the value of the upperpage data state is not affected whether or not the lower page data statewas altered according to various embodiments of the present disclosure.That is, the upper page data value is neither inverted when writing tothe memory device nor when reading from the memory device.

Without having the benefit of the methods according to embodiments ofthe present disclosure, the example illustrated in FIG. 6 would resultin 141 (summation of column 614) memory cells programmed to the 00 or 10states. FIG. 7 illustrates that utilizing the embodiments of methodsaccording to the present disclosure, a total of 49 (summation of column724) memory cells would be programmed to the 00 or 10 state as opposedto 141 memory cells as shown in FIG. 6. This significant reduction inthe number of memory cells undergoing a lower page programming operationcan reduce the number of memory cells being programmed to the higherdata states such as 308 and 310 as shown in FIG. 3, for example. Thisresults in a fewer number and lower magnitude of programming pulses thatneed to be utilized during a programming operation which thereby canreduce the possibility of program disturb, reduce gate stress on thememory cells and/or reduce the time required to perform the programmingoperation. One or more of the embodiments of the present disclosureserve to redistribute the threshold voltages of a page of memory cellsand not simply invert all the data states of the memory cells. Themethod of the '868 patent referenced above, in the case where a decisionhas been made to invert the data, results in memory cells that were toremain erased to now be programmed and memory cells originally to beprogrammed to now be held in an erased state. The embodiments of thepresent disclosure are not so limited.

The benefit of the embodiments of the present disclosure can also beillustrated with reference to TABLE 1. Table 1 illustrates a worst casescenario for the final programmed data levels (e.g., distributions) foran example population of 100,000 memory cells and a 50% threshold countvalue. The conclusions of Table 1 also applies to memory cellpopulations of different numbers of memory cells. The worst casescenario for the embodiments of the present disclosure utilizing a 50%threshold count value is 50% of the memory cells will be programmed tothe 01 state and 50% will be programmed to 10 state. This is in contrastwith the worst case scenario of 100% of the memory cells residing in the10 state when the methods of the present embodiments are not utilized.The worst case scenario illustrated by Table 1 for the prior art methodof writing data as received without data conditioning would be all100,000 memory cells programmed to the 10 state. However, according toone or more embodiments of the present disclosure, having more than50,000 cells with an initially intended data state calling for a lowerpage programming operation would result in an inversion of the lowerpage data assuming a greater than 50% threshold count value. Thus, theworst case scenario for utilizing data conditioning methods ofembodiments of the present disclosure would be a situation wherein halfof the memory cells are to be programmed to the 10 state and half to the01 state.

TABLE 1 Worst Case Final Programmed Data States Final ProgrammedLevel/Data State Method Utilized 11 01 00 10 Without Data 0 0 0 100,000Conditioning With Dynamic 0 50,000 0  50,000 MLC Data State Conditioning

Additionally, the result of the count operation (e.g., summation of 1sor 0s) performed can also be utilized to determine an initialprogramming voltage for a page of memory cells. As discussed above,flash memory cells are programmed by applying a sequence of everincreasing voltage level pulses, typically up to some particularsaturation level, to a word line until all of the memory cells coupledto the word line have reached their desired state. Different memorycells may require a different amount and magnitude of programming pulsesto reach the same state. For example, a group of memory cells may all beprogrammed to the same state. Some of the group may require a 16Vprogramming voltage wherein other cells may require an 18V programmingvoltage. Thus, if a relatively low number of cells require a lower pagedata programming operation, a lower initial programming voltage can beused. As the number of memory cells requiring a lower page programmingoperation increases, statistically speaking it is more likely that someof the memory cells would benefit from a higher initial programmingvoltage. Thus, as the number of memory cells requiring a lower pageprogramming operation increases, the initial programming voltage levelshould also be increased. This results in an increase in programmingefficiency and less of a chance of program disturb because in somesituations lower programming voltages are utilized. Thus, the initialprogramming potential can be, at least in part, a function of the count(e.g., Initial_Programming_Voltage=f(count)). Thus, utilizing the resultof a summation of 1s or 0s operation discussed above to determine aninitial programming voltage for a page of memory cells increasesefficiency and reduces the possible effects of programming disturb. Forexample, the function may yield the initial programming pulse conditionsshown in Table 2. Other embodiments consider the count to not onlyadjust the magnitude of the programming potential pulses but also toadjust the duration (e.g., pulse width) of each programming pulse.Therefore, it is a further embodiment of the present disclosure that thecharacteristics of the initial applied programming waveform can be afunction of the summation of 0s (or 1s) result.

TABLE 2 Initial Programming Pulse Conditions Number of lower pageInitial 0s to program (% of Programming memory cells) Pulse Voltage    0-10% 12 V 11%-20% 12.5 V   21%-30% 13 V 31%-40% 13.5 V   41%-50% 14V

FIG. 8 illustrates an internal data move (IDM) according to one or moreembodiments of the present disclosure. This can include data that hasbeen conditioned according to various embodiments of the presentdisclosure and might have an inversion flag associated with it, forexample. Such a data move operation can take place internal to thememory device without interaction from a host coupled to the memorydevice. The internal data move operation can be effectuated by memorydevice control logic (e.g., control circuitry) such as that discussedsubsequently with respect to FIG. 9. Upon the generation of the commandto perform the internal data move operation 800, a determination is made802 to determine if the data to be moved needs to be read from an upperor a lower page. If the data is to be read from an upper page 804, aupper page data read operation is performed and the read data is putinto a data cache 808. If the internal data move operation is to readdata from a lower page 806, a read operation of the lower page data isperformed and the lower page data is put into the data cache 810. Acheck is performed 812 to determine if the data read from the lower pagehas an inversion flag associated with it. If the lower page data has ainversion flag that has been set 816 (e.g., the data was previouslyinverted), an inversion operation is performed on the lower page readdata 818. If an associated inversion flag has not been set 814, noinversion operation is performed on the lower page read data. Accordingto various embodiments of the present disclosure, the inversion flag maybe read prior to, concurrently with, or subsequent to the read operation810, for example. Once data, either upper or lower page data, is putinto the data cache, additional processing (e.g., modification) of thedata may occur 820. This additional processing may occur at thedirection of a host coupled to the memory device or may be an internalmemory device operation, for example.

Upon completion of any data modification of data residing in the datacache 820, a decision is made 822 to determine if the destination of thedata residing in the data cache is to be written to an upper 824 or alower 826 page location. Data having an upper page destination 824 isthen written 828 to the intended upper page location of memory. Datahaving a lower page destination 826, is subjected to a count operation830 such as count operation 404 described with respect to FIG. 4 above.If the count is equal to, or exceeds, some particular threshold countvalue 832/836, an inversion operation is performed on the data and itsassociated inversion flag bit is set 838. If the count is less than theparticular threshold count value 834, no inversion is performed on thedata. A programming operation is then performed of the data to the lowerpage destination in memory 840.

FIG. 9 is a functional block diagram of an electronic system having atleast one memory device 900 according to an embodiment of the presentdisclosure. The memory device 900 is coupled to a processor 910. Theprocessor 910 can be a microprocessor or some other type of controllingcircuitry. The memory device 900 and the processor 910 form part of anelectronic system 920. The memory device 900 has been simplified tofocus on features of the memory that are helpful in understandingvarious embodiments of the present disclosure.

The memory device 900 includes an array of memory cells 930 that can bearranged in banks of rows and columns. For example, the memory array canbe an array of flash memory cells arranged in a NAND configuration.

Row decode circuitry 944 and column decode circuitry 950 are provided todecode address signals. Address signals are received and decoded toaccess memory array 930. Memory device 900 also includes input/output(I/O) control circuitry 960 to manage input of commands, addresses anddata to the memory device 900 as well as output of data and statusinformation from the memory device 900. An address register 940 iscoupled between I/O control circuitry 960, row decode circuitry 944 andcolumn decode circuitry 950 to latch the address signals prior todecoding. A command register 948 is coupled between I/O controlcircuitry 960 and control logic 970 to latch incoming commands. Controllogic 970 controls access to the memory array 930 in response to thecommands and generates status information for the external processor910. The control logic 970 is coupled to row decode circuitry 944 andcolumn decode circuitry 950 to control the row decode circuitry 944 andcolumn decode circuitry 950 in response to the addresses. Control logic970 also comprises in part, various structures and circuits in order tofacilitate implementation of various embodiments of the presentdisclosure. For example, control logic 970 can include a state machineand/or various logic circuitry and control registers.

Control logic 970 is also coupled to one or more registers such as cacheregister 952 and/or data register 946. According to one or moreembodiments of the present disclosure, the data cache described abovewith respect to FIG. 8 can be comprised of the cache register 952 and/orthe data register 946, for example. Cache register 952 latches data,either incoming or outgoing, as directed by control logic 970 totemporarily store data while the memory array 930 is busy writing orreading, respectively, other data. During a write operation, data ispassed from the cache register 952 to data register 946 for transfer tothe memory array 930; then new data is latched in the cache register 952from the I/O control circuitry 960. During a read operation, data ispassed from the cache register 952 to the I/O control circuitry 960 foroutput to the external processor 910; then new data is passed from thedata register 946 to the cache register 952. The cache register 952 andthe data register 946 may comprise various analog (e.g., sense devices)and/or digital circuitry as are known to those skilled in the art. Astatus register 956 is coupled between I/O control circuitry 960 andcontrol logic 970 to latch the status information for output to theprocessor 910.

Memory device 900 receives control signals at control logic 970 fromprocessor 910 over a control link 972. The control signals present onthe control link 972 may include a chip enable CE#, a command latchenable CLE, an address latch enable ALE, a write enable WE#, a readenable RE# and a write protect WP# signal. Memory device 900 receivescommands (in the form of command signals), addresses (in the form ofaddress signals), and data (in the form of data signals) from processor910 over a multiplexed input/output (I/O) bus 962 and outputs data toprocessor 910 over I/O bus 962.

Specifically, the commands are received over input/output (I/O) pins[7:0] of I/O bus 962 at I/O control circuitry 960 and are written intocommand register 948. The addresses are received over input/output (I/O)pins [7:0] of bus 962 at I/O control circuitry 960 and are written intoaddress register 940. The data are received over input/output (I/O) pins[7:0] for an 8-bit device or input/output (I/O) pins [15:0] for a 16-bitdevice at I/O control circuitry 960 and are written into cache register952. The data are subsequently written into data register 946 forprogramming memory array 930. For another embodiment, cache register 952may be omitted, and the data are written directly into data register946. Data are also output over input/output (I/O) pins [7:0] for an8-bit device or input/output (I/O) pins [15:0] for a 16-bit device 962.It will be appreciated by those skilled in the art that additionalcircuitry and signals can be provided, and that the memory device ofFIG. 9 has been simplified to help focus on the present disclosure.Additionally, while the memory device of FIG. 9 has been described inaccordance with popular conventions for receipt and output of thevarious signals, it is noted that various embodiments of the presentdisclosure are not limited by the specific signals and I/Oconfigurations described unless expressly noted herein.

Conclusion

Memory devices and methods have been described capable of providing dataconditioning of lower page data values in order to statistically reducethe number of memory cells that are programmed to higher thresholdvoltage ranges. This statistical reduction in memory cells programmed tohigher threshold voltage ranges facilitates benefits of reduced gatestress in the memory devices, reduced programming time, a reduction inprogram disturb effects, an increase in reliability and/or a reductionin the number and magnitude of programming pulses applied to memorycells of the memory device.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe disclosure will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the disclosure.

1. A method of operating a multi state memory device, comprising:determining whether a majority of a plurality of memory cells are to beprogrammed to threshold voltages above an intermediate point of allthreshold voltages used to represent possible data states of the memorydevice; and if a majority of the plurality of memory cells are to beprogrammed to threshold voltages occurring above the intermediate point,programming the majority of the memory cells to threshold voltages belowthe intermediate point.
 2. The method of claim 1, wherein determiningwhether a majority of a plurality of memory cells are to be programmedto threshold voltages above an intermediate point of all thresholdvoltages used to represent possible data states of the memory devicecomprises determining whether a majority of a page of memory cells areto be programmed to threshold voltages above an intermediate point ofall threshold voltages used to represent possible data states of thememory device.
 3. The method of claim 1, wherein the intermediate pointis a midpoint.
 4. The method of claim 2, further comprising setting aflag associated with the page of memory cells if the majority of thepage was to be programmed to threshold voltages above the intermediatepoint.
 5. The method of claim 2, wherein determining whether a majorityof a page of memory cells are to be programmed to threshold voltagesabove an intermediate point of all threshold voltages used to representpossible data states of the memory device comprises performing acumulative current measurement of currents indicative of data to bewritten to the page of memory cells and performing a comparison of thecumulative current measurement and a reference current.
 6. A method forprogramming a page of memory cells wherein each memory cell stores aplurality of digits comprising data, the method comprising: determiningwhether a count of memory cells of the page of memory cells that wouldalter a first digit of their respective data in response to theprogramming is greater than a threshold count value; inverting the firstdigit of the data of each memory cell of the page of memory cells andsetting a flag bit indicating the first digits of the data of eachmemory cell has been inverted when the count of memory cells of the pageof memory cells is greater than the threshold count value; andprogramming the first digit of the data of each memory cell of the pageof memory cells.
 7. The method of claim 6 wherein the first digit of thedata value of a memory cell comprises a lower page data value of amemory cell and a second digit of each data value comprises an upperpage data value of a memory cell.
 8. The method of claim 6 whereinprogramming comprises applying at least one programming pulse having apulse potential and a pulse duration to the page of memory cells toalter a threshold voltage of at least one memory cell.
 9. The method ofclaim 8 wherein the pulse potential of the first applied programmingpulse is a function of the count corresponding to memory cells havingtheir first digit altered.
 10. The method of claim 8 wherein each pulseduration is a function of the count corresponding to memory cells havingtheir first digit altered.
 11. The method of claim 6, furthercomprising: performing a second programming to program a second digit ofthe data of each memory cell of the page of memory cells, wherein thesecond programming is performed without inverting the second digit ofthe data of any memory cell of the page of memory cells regardless of acount of the memory cells of the page of memory cells that would alterthe second digit of their respective data in response to the secondprogramming.
 12. The method of claim 6 wherein the threshold count valueis greater than approximately 50% of the page.
 13. A method ofprogramming a page of multilevel memory cells of an array of memorycells arranged in a NAND configuration, the method comprising: during afirst programming operation of the page of memory cells, if a firstnumber of memory cells of the page of memory cells having a data valueindicating a desire to shift their data states is greater than a secondnumber of memory cells of the page of memory cells having a data valueindicating a desire to leave their data states in an erased state,programming the page of memory cells to shift the data states of thesecond number of memory cells and to leave the data states of the firstnumber of memory cells in the erased state, and setting a flag toindicate that the data states of the memory cells of the page of memorycells have been altered from their initially intended data states; andduring a second programming operation of the page of memory cells,programming the page of memory cells according to data values for thesecond programming operation and the data states from the firstprogramming operation.
 14. The method of claim 13 wherein a programmingoperation comprises applying at least one programming pulse having aprogramming potential to the page of memory cells, and wherein theprogramming potential increases for each successive programming pulseapplied to the page of memory cells.
 15. The method of claim 14 whereina magnitude and a duration of the first applied programming pulse is afunction of the number of memory cells indicating a desire to shifttheir data states.
 16. A memory device, comprising: a plurality ofmultilevel memory cells comprising a page of memory cells; inputcircuitry configured to receive external data comprising a plurality ofdata values each data value to be stored in a different memory cell ofthe page of memory cells, wherein each data value comprises a pluralityof digits; and control circuitry configured to determine whether amajority of memory cells of the page of memory cells are to beprogrammed to threshold voltages above an intermediate point of allthreshold voltages used to represent possible data states of the memorydevice and, if a majority of the memory cells of the page of memorycells are to be programmed to threshold voltages occurring above theintermediate point, programming the majority of the memory cells tothreshold voltages below the intermediate point.
 17. The memory deviceof claim 16 wherein the control circuitry is further configured toperform a cumulative current measurement while storing data to bewritten to the page of memory cells and performing a comparison of thecumulative current measurement and a reference current to determine acount of memory cells to be programmed to threshold voltages above theintermediate point.
 18. The memory device of claim 16 wherein thecontrol circuitry is further configured to utilize a digital adder todetermine a count of memory cells to be programmed to threshold voltagesabove the intermediate point.
 19. The memory device of claim 16 whereinthe control circuitry is further configured to alter lower page digitsof data values stored in the page of memory cells in response to theflag bit as part of a memory read operation.
 20. The memory device ofclaim 19 wherein an alteration of lower page digits comprises aninversion operation performed on each lower page digit of the pluralityof data values prior to output of the data from the memory device.
 21. Amemory device, comprising: a plurality of non-volatile memory cellscomprising a page of memory cells having commonly coupled control gates,wherein each memory cell of the page is coupled to a different data lineand wherein each memory cell is configured to store data comprising anupper and a lower page; a storage location for an inversion flag; andcontrol circuitry configured to count data values of a lower page ofdata having a first data state to be written to the page of memorycells; and wherein the control circuitry is further configured to alterthe data state of the lower page of data to be written to each memorycell of the page of memory cells if the count of first data statesexceeds a threshold count value, set the inversion flag and perform awrite operation of the lower page of data to the page of memory cells.22. The memory device of claim 21 wherein the control circuitry isfurther configured to apply a programming pulse to the plurality ofmemory cells wherein a magnitude and a duration of the programming pulseis a function of the number of memory cells to be written to the firstdata state.
 23. The memory device of claim 21 further comprising a datacache configured to store data to be written to each memory cell and togenerate a plurality of currents in response to the data stored in thedata cache.
 24. The memory device of claim 23 wherein the controlcircuitry is further configured to generate a cumulative currentmeasurement comprising a summation of the plurality of currents andperform a comparison of the cumulative current measurement and areference current.
 25. The memory device of claim 21 wherein the controlcircuitry is further configured to perform an internal data moveoperation of data stored in a first memory location to a second memorylocation of the memory device.